# code 1-1
# todo:注释每一行的意思
print('Welcome to the GPA calculator.')
print('Please enter all your letter grades, one per line.')
print('Enter a blank line to designate the end.')

points = {'A+': 4.0, 'A': 4.0, 'A-': 3.67, 'B+': 3.33, 'B': 3.0, 'B-': 2.67,
          'C+': 2.33, 'C': 2.0, 'C-': 1.67, 'D+': 1.33, 'D': 1.0, 'F': 0.0
          }

num_courses = 0
total_points = 0
done = False

while not done:
    grade = input()  # read line from user
    if grade == '':
        done = True
    elif grade not in points:
        print("Unknown grade '{0}' being ignored".format(grade))
    else:
        num_courses += 1
        total_points += points[grade]

    if num_courses > 0:
        print('Your GPA is {0:.3}'.format(total_points / num_courses))

